The present invention relates to a drilling fluid containing additives used for stabilizing muddy water. The drilling fluid is used in cases where the ground is deeply excavated, such as a foundation work in engineering works, boring of a petroleum well or a geothermal well, and so on.
Hitherto, in cases where the ground is excavated, drilling fluid prepared on the ground surface is continuously and cyclically injected into the excavating parts to absorb frictional high heat generated during excavation, to move excavation trash from the pit bottom toward the ground surface, to maintain the drill pit wall, or to prevent collapse of a layer which is undergoing degradation.
The drilling fluid is usually prepared in a muddy state using inorganic clayey substances such as bentonite, attapulgite, and sericite, with water, and also with drilling fluid additives to stabilize the drilling fluid.
Among inorganic compounds used as drilling fluid additives, condensed phosphates such as sodium tripolyphosphate and sodium hexametaphosphate; alkali-metal carbonates such as sodium carbonate and potassium carbonate; aluminates such as sodium aluminate and potassium aluminate are known. Among organic compounds used as drilling fluid additives, humic acid salts, lignosulfonic acid salts, carboxymethylcellulose (CMC), hydroxyethylcellulose (HEC), polyvinylalcohol (PVA), and phosphonic acids are known. However, a lot of problems have been reported with the use of such additives.
Although, relatively speaking, the condensed phosphates have been highly evaluated, if their use is continued for a long period, they spread over the ground and flow into seas or lakes. They may, therefore, cause the generation of water-bloom or red tide. As such, their use in recent years has been strictly limited. Although CMC has gotten a relatively favorable evaluation, drilling fluid used with it easily decomposes posing a serious problem. Low effects are shown by alkali-metal carbonates, aluminates, humic acid salts, lignosulfonic acid salts, HEC, PVA, and phosphonic acids.
To solve the above-described problems for said additives hitherto used, there has been proposed the use of polyacrylic acids (salts) as drilling fluid additives in Japanese Official Patent Provisional Publications; showa 55-104383, 58-84883, 58-208376, 58-208377, 59-135280, 60-133085, and 62-199682.
However, since the polyacrylic acids or salts are not satisfactory in salt-resisting properties, in cases where water abundant in water hardness component or sea water is used as an aqueous medium to prepare drilling fluid, or in cases where plenty of underground water or sea water flows into drilling fluid in the underground working parts, the viscosity of the drilling fluid suffers damage and evacuation sometimes becomes difficult because of resulting gelation.
There has been proposed a method to remove the problems associated with the use of polyacrylic acids or salts. For example, Japanese Official Patent Provisional Publications, showa 58-120683, 58-219289, 60-181187, 61-120881, and 62-267388, have proposed drilling fluid additives composed of polyacrylic acids or salts in joint use with other components. While these additives have been somewhat, but insufficiently, improved in salt-proof properties, their use in relatively large amount is still needed. As such, it is hard to say whether this substantial fault in polyacrylic acids or salts has been removed. There has also been proposed, in Japanese Official Patent Provisional Publications, showa 58-104981, 61-241382, and 62-215681, the use of copolymers containing a sulfonic group. While copolymers containing a sulfonic group, when compared with polyacrylic acids or salts, have advantages such as improved salt-resisting properties, problems still exist. In this regard, although copolymers containing a sulfonic group described in Japanese Official Patent Provisional Publication, showa 58-104981, have been formed among an .alpha., .beta.-ethylenic unsaturated carboxylic acid monomer, a monomer having a sulfonic group (vinylsulfonic acid, (meth)allylsulfonic acid, styrenesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, and sulfoethyl methacrylate) and a (meth)acryl acid ester monomer or an unsaturated nitrile monomer, all of these still have the problem that if the mixing percentage of sea water exceeds 50%, the effect much lowers. Although the copolymers containing a sulfonic acid described in Japanese Official Patent Provisional Publication, showa 61-241382, have been formed between a (meth)acrylic acid monomer and a sulfoalkyl (meth) acrylate monomer, all of these have a high polymerization degree, so that the viscosity of the drilling fluid becomes high. Although the copolymers containing a sulfonic group described in Japanese Official Patent Provisional Publication, showa 62-215681, have been formed among (meth)acrylamide, a defined anionic vinyl monomer such as (meth)acrylic acid salts, 2-acrylamido-2-methylpropanesulfonic acid salts, and a defined water-soluble amphoteric vinyl monomer having a sulfonic group such as N-(2-sulfoethyl)-N-acrylamidoethyl-N,N-dimethylammonium amphoteric monomer and N-(3-sulfopropyl)-N-methacrylamidopropyl-N,N-dimethylammonium amphoteric monomer, all of these cause lowering of the effect if mixing percentage of sea water exceeds 60%.